This invention pertains to solid state electrolyte cells for use in fluid separation devices and fuel cells. Fluid separation devices and fuel cells utilize solid oxide ceramic materials as ionic electrolytes or membranes for transporting ions of certain substances thereby enabling electrical current to flow through such apparatuses. When such apparatuses are used as fluid separation devices a fluid, for example air, is introduced to one side of the solid ceramic membrane across which an electrical potential has been applied. The oxygen is ionized and oxygen ions are transported through the membrane. On the opposite side of membrane, the oxygen ions recombine to form gaseous oxygen and free electrons. When such apparatuses are used as fuel cells, a fluid reactant, for example hydrogen, is introduced to one side of the solid ceramic membrane and another fluid reactant, for example air or oxygen, is introduced to the other side of the membrane. The oxygen is ionized and oxygen ions are transported through the membrane whereupon the oxygen ions react with the hydrogen forming H.sub.2 O and releasing electrons thereby generating an electrical potential and current.
Examples of fluid separation devices utilizing solid ceramic membranes are disclosed in U.S. Pat. No. 4,885,142, which is hereby incorporated herein by reference. This patent discloses several devices for separating oxygen from air in which air distributed at the periphery of a disk of solid ceramic membrane, flows radially inwardly over the disk as oxygen ions are transported through the membrane disk. Air having a reduced oxygen content is removed along the axis of the membrane disk. Stacks of cells are used to improve the separation of oxygen from air. In one stack of cells arrangement, alternating membrane disks and ceramic distribution plates are used. The ceramic distribution plates have radially extending internal ports for distributing air from the periphery of the membrane disk radially inwardly towards the axis. In this arrangement, the electrical current is distributed circumferentially around the edge of the membrane disk by a platinum wire contact ring and electrons flow radially inwardly. Unfortunately, it has been found that the construction of distribution plates having radially extending or horizontal internal ports which are completely internal of the distribution plate is very difficult especially if thin distribution plates are required or desired. In another stack of cells arrangement, the membrane disks are separated by layers of porous packing material made from particles of electrically conductive ceramics such as strontium-doped lanthanum manganite. In this arrangement, electrical current flows axially through the membrane disk. Unfortunately, the layers of packing ceramic material increase the electrical resistance, weight and height of the apparatus which is not desirable.
Methods of producing improved solid ceramic membranes useful for transporting ions are disclosed in U.S. Pat. No. 4,957,673, which is hereby incorporated herein by reference. This patent discloses a method of producing a solid ceramic electrolyte material having a high density electrolyte sandwiched between layers of solid ceramic electrode material having a low density porous structure. One embodiment comprises a sintered central layer of yttria stabilized zirconia ("YSZ") having 95% or more theoretical density and about 8 mole percent yttria, and sintered outer layers of strontiumdoped lanthanum manganite ("LSM") having interconnected porosity and density of 50 to 60% of theoretical with the approximate composition La.sub.0.8 Sr.sub.0.2 MnO.sub.3. The LSM/YSZ/LSM sandwich structure permits the flow of air through the first layer of porous LSM. Oxygen is then ionized and the oxygen ions are transported through the central YSZ layer. At the other outer layer of porous LSM, oxygen ions recombine and release electrons. The interconnected porous structure of the LSM layers permits the flow of gases to and from the interface of the YSZ-to-LSM layers. The membranes disclosed in U.S. Pat. No. 4,957,673 are especially useful in the cells of the invention described hereinafter.
A distribution plate made from A-lava without radially extending internal ports is disclosed in an article entitled OXYGEN SEPARATION FROM AIR USING ZIRCONIA SOLID ELECTROLYTE MEMBRANES, Proceedings of the 23rd Intersociety Energy Conversion Conference, Vol. 2, ASME, New York, 1988, pp. 273-277. The thickness of the A-lava distribution plates was relatively large compared to the thickness of the membrane disks. Thick distribution plates are a disadvantage if thin electrolyte cells are required or desired. Reference is also made to a ribbed design that contacts the electrode surface for current flow, that provides support for the thin zirconia membrane, and that directs the gas flow from the outer circumference to the center of the disk. The ribbed distribution plates also contained horizontal internal ports for flowing air, oxygen, and oxygen-depleted air in and out of the separation chamber. Unless otherwise specified "horizontal" means parallel to the plane of the member, and "vertical" means perpendicular to the plane of the member. By the term "horizontal internal port" as used herein is meant a port entirely within the member and parallel to the plane of the member.
To minimize the overall height and resistance of apparatuses employing stacks of cells, there is a need to reduce the thickness of all disk members and particularly members containing ribs or horizontal internal ports. Since low temperature electrodes with ribs can be made of metals, the electrodes can be made very thin. However, where the apparatus is required to operate at very high temperatures, as is the case for many separators and fuel cells, such high temperatures are generally beyond the useful temperature range for metals. Although platinum can be used at high temperatures, its cost essentially precludes its use even if applied in only a thin layer. In high temperature cells, both the electrode members and the membrane members are generally made from ceramic materials. Thus, there is a need for a process for producing thin members from ceramic materials.
What is needed is a thin distribution plate made from a solid ceramic electrode material which is suitable for use in an electrolyte cell and which will support an electrolyte membrane in a stack of cells. It is also desirable that the distribution plate provide for parallel porting of fluids to and from the cells and for serial flow of electrical current axially through each cell. Since internal horizontal passageways completely within a distribution plate tend to weaken it and cause it to break or fail at such passageways, it is also desirable that the distribution plate not contain any internal horizontal passageways.